This invention relates to a process for removing phenols from waste water from the gasification or carbonisation of coal, comprising extracting the waste water with a low-boiling organic solvent, recovering the solvent, removing the acid constituents under superatmospheric pressure, and stripping off the ammonia.
The waste water occurring during pressure gasification or carbonisation of coal contains phenols, fatty acids, neutral oils as well as dissolved gases, such as ammonia, carbon dioxide, hydrogen sulfide, and hydrogen cyanide.
Numerous processes are known in which phenols are removed from such waste water by an extraction with low-boiling solvents.
For instance, it is known to remove phenols from ammonia-containing waste water from coke oven plants, carbonisation plants, gas producing plants and the like by a process in which the waste water is intimately mixed with an oxygen-containing low-boiling organic solvent, such as butyl acetate. In the subsequent stratification, a major part of the phenols enters the solvent. When the process is repeated several times in a counterflow operation, the phenols can be recovered in a yield above 99.8%. The organic solvent required to extract the phenols may be purified by distillation for subsequent re-use.
The dephenolised water may be treated to remove ammonia, if desired, and may then be subjected to a biological afterpurification (Lurgi-Manual, 1960, pages 64/65).
It is also known to use two solvent cycles for dephenolising industrial waste water. A major part of the phenols is removed in the first cycle and the remainder in the second. The solvent of the first cycle is removed from the laden solvent of the second cycle, and the solvent recovered by distillation from the first cycle is fed to the second cycle (German Pat. Specification No. 939,268). This process is applicable only to two gas liquor streams which differ in phenol distribution and rate.
After each dephenolising process, it is necessary to remove the residual substances still contained in the waste water, particularly hydrogen sulfide, carbon dioxide, and ammonia, and the organic solvent used for extraction. Various processes have been proposed for the removal of hydrogen sulfide, carbon dioxide, and ammonia.
U.S. Pat. No. 2,018,863 describes a process for recovering ammonia from gases produced by the distillation of coal. In that process, the gases are first cooled to 35.degree. C. to form a condensate which contains ammonia and smaller quantities of CO.sub.2 and H.sub.2 S. This condensate is fractionated and the gases are scrubbed to remove impurities, particularly CO.sub.2 and H.sub.2 S. The scrubbed gas consists mainly of ammonia. In that process, the scrubbed gas must be scrubbed with sulfuric acid to remove the residual water vapor. Because the cooling is so controlled that the resulting ratio of ammonia to dissolved CO.sub.2 and H.sub.2 S is at least 2:1, the process can be used to process only gases in which ammonia predominates.
Gases produced by distillation of coal can also be scrubbed with a slightly ammoniacal aqueous solution, the spent scrubbing solution being distilled at low temperature and under a low pressure to remove H.sub.2 S and HCN and smaller quantities of CO.sub.2. The remaining spent scrubbing solution containing ammonium carbonate is distilled to remove H.sub.2 S,HCN and ammonia. These removed substances are condensed and the condensate is fractionated to remove ammonia. The residual solution is recycled to the scrubber (U.S. Pat. No. 2,162,838).
If the ammonia is not to be recovered but to be burnt, hydrogen sulfide must be separately removed, for ecological reasons. Besides, the ammonia must be present in the highest concentration possible, i.e., the quantity of accompanying steam must be minimized so that its calorific value is utilized as fully as possible.
A high CO.sub.2 content of these waste waters is not desirable because carbonate and carbamate tend to precipitate in cold conduit portions from gases which contain ammonia and CO.sub.2 and small amounts of accompanying steam. Such precipitates will greatly disturb or may even prevent the operation. It is obvious to accomplish these objects as follows:
(a) The solvent is stripped off with circulated gas; the solvent-laden circulated gas is then scrubbed with cooled raw phenol; solvent is jointly stripped by means of stripping steam from the circulated phenol and from the raw phenol.
(b) This is followed by a pressure de-acidification to remove about 50-70% of the CO.sub.2 and about 98-99% of the H.sub.2 S. The mixed CO.sub.2 and H.sub.2 S gases are scrubbed with cold water to remove NH.sub.3.
(c) Ammonia is stripped off together with the residual CO.sub.2 and H.sub.2 S.
It is also known that a higher boiling solvent, such as n-butyl acetate, may be used and may be recovered from the refined product in that the solvent vapors which have been stripped off under atmospheric pressure and which become available in a mixture with NH.sub.3, CO.sub.2, H.sub.2 S and H.sub.2 O, are scrubbed twice, one each
(a) with 50% of the cold refined product and PA1 (b) with 100% of the incoming waste water, which has been cooled and still contains phenols ("Ol und Kohle" 38 (1942), 525; "Erdol und Kohle" 4 (1951), 177-180).
This procedure involves a circulation of NH.sub.3, CO.sub.2 and H.sub.2 S gases at a high rate through the entire plant, and the escaping acid gas still contains relatively large quantities of NH.sub.3 and solvent so that the solvent losses are high. Besides, n-butyl acetate suffers large losses by saponification, and NH.sub.3 has a disturbing influence during the further processing of the mixed CO.sub.2 and H.sub.2 S gases or their destruction.
It is accordingly an object of the invention to avoid these and other disadvantages of the known processes and to provide a process which is simple and has a high efficiency.
This object is accomplished according to the invention in that the solvent dissolved in the refined product is stripped off under superatmospheric pressure jointly with the carbon dioxide and hydrogen sulfide, the resulting vapors are scrubbed under superatmospheric pressure with a small part of the cold refined product, part of the solvent is recovered by being condensed and by being separated under superatmospheric pressure, and the remaining acid gases, which still contain ammonia and solvent, are scrubbed with circulating, cooled raw phenol in order to remove residual ammonia and residual solvent.